Analog Signal Mystery!!!!

I am stumped on this one and looking for some help.

I am working on a Modicon 984 processor. I recently added an 875-111 differential analog input card. The card is monitoring 2 chlorine and pH meters and 2 turbity meters. I am scaling the raw data in the SCADA software(Wonderware). The output for the turbity is perfect. I've tested the channels with a 4-20mA generator and the scaling is bang on. I've added these exact pH and chlorine meters to a separate 984 PLC without issue. Here is the problem:

When I connect the pH/Chlorine meter to the PLC the reading on the SCADA screen and the LCD on the unit do not jive. When I put a meter in series with the meter and the PLC, the 4-20mA signal and the SCADA screen perfectly jive. However, when I disconnect meter from the PLC and connected a 4-20mA reader to the wire leads of the meter by itself the 4-20mA signal is different (larger) and corresponds to the LCD display on the unit itself. Example: The unit has a 4-20mA test current function. When I put it to each of the settings with just the reader attached I get a perfect signal (19.97mA when on 20mA). However, when I wire it back up to the PLC with a meter in series there appears to be a distortion/or loss (16.85mA when on 20mA). It does not appear to be linear.

Also, note that the register in the PLC (0-4095) jives with what is being see with the meter (hence the scaling working). The meters have be calibrated and no offsets appear to be programmed.

Any help or suggestions would be greatly appreciated.

Tim

Is it wired correctly? Wiring a single ended transmitter is different then wiring a differential transmitter. Is the system grounded correctly

What is the load specification of your meter? Is that load specification able to drive the Modicon input card. (This is a shot in the dark because I'm not at work so I don't have my Modicon specs) For example your meter may be rated for a 250 ohm max load, but if the Modicon input is 500 ohms then the meter cannot output the voltage necessary to push 20mA through a 500 ohm load. Also make sure that your instrument power supply is adequate for the PLC input. A 5V power supply will never be able make a 500 ohm input reach 20mA. If the power supply is a 12V or 24V or +/-15V power supply then check that its is outputting the correct voltage when the meter is attached.

I'm guessing that you have a ground loop in the wiring - when the signals are tied together you are getting "cross-talk" between channels.

Try disconnecting all analog loops, then connecting just one. If the reading is OK, disconnect this sensor and connecting just the next one. Repeat as required. If the meters all check out OK individually then start with one, add a second, add a third, etc. until you no longer get good readings.

If the adding the second meter causes readings to be inaccurate, remove it and put in just the first third. Repeat as required.

This will tell you where the problem lies.

Note that I expect at least the turbidity to be self powered (four wire) transmitters. The pH may be loop powered (two wire) or self powered. These need to be wired differently.

FYI: If anyone is interested the pH/Chlorine transmitter is:

Grundfos Conex DIA-1, from manual:
"4 analog outputs 0(4)-20mA, feely adjustable, max load 500ohm"

From Modicon literature:
"Input resistance: current mode: 250ohm, +/-0.05%"

Thanks for the input I will be looking into these.

Tim

Let me restate what you've reported, as 4 discrete situations:

timothywhitson said:

When I connect the pH/Chlorine meter to the PLC the reading on the SCADA screen and the LCD on the unit do not jive.

Click to expand...

1) analyzer LCD - PLC input - SCADA screen

LCD ≠ SCADA

When I put a meter in series with the meter and the PLC, the 4-20mA signal and the SCADA screen perfectly jive.

Click to expand...

2) analyzer LCD - milliamp meter - PLC input - SCADA screen

LCD = SCADA

However, when I disconnect meter from the PLC and connected a 4-20mA reader to the wire leads of the meter by itself the 4-20mA signal is different (larger) and corresponds to the LCD display on the unit itself. Example: The unit has a 4-20mA test current function. When I put it to each of the settings with just the reader attached I get a perfect signal (19.97mA when on 20mA).

Click to expand...

3) analyzer LCD - milliamp meter

analyzer LCD = 4-20mA meter reading for test current outputs (perfect 19.97 = 20.00)

However, when I wire it back up to the PLC with a meter in series there appears to be a distortion/or loss (16.85mA when on 20mA). It does not appear to be linear.

Click to expand...

4) analyzer LCD - milliamp meter - PLC input - (SCADA screen)

distortion/loss; 16.85mA when on 20mA, not linear


As I understand it, Situation #4 is identical to situation #2, but you do not report the same results.

Situation #2: LCD = SCADA
Situation #4: low reading

Which is the result for
analyzer LCD - milliamp meter - PLC input - SCADA screen
Low reading or on the money?
Dan

timothywhitson said:

FYI: If anyone is interested the pH/Chlorine transmitter is:

Grundfos Conex DIA-1, from manual:
"4 analog outputs 0(4)-20mA, feely adjustable, max load 500ohm"

From Modicon literature:
"Input resistance: current mode: 250ohm, +/-0.05%"

Thanks for the input I will be looking into these.

Tim

Click to expand...

Then that doesn't appear to be the problem. A 4-20mA can usually drive a load that is lower that its rated max.

Next I'd recommend looking for a ground loop as Tom suggested.

timothywhitson said:

FYI: If anyone is interested the pH/Chlorine transmitter is:

Grundfos Conex DIA-1, from manual:
"4 analog outputs 0(4)-20mA, feely adjustable, max load 500ohm"

From Modicon literature:
"Input resistance: current mode: 250ohm, +/-0.05%"

Thanks for the input I will be looking into these.

Tim

Click to expand...

'Max load 500 ohm' means 'minimum resistance = 500 ohms'. Connect Grunfos directly to PLC to verify (edit - test #1 did verify this).
Use an analog input. Set it for 2-10 volts. Connect a precision 500 ohm resistor across it. This will give you a 4-20mA input with 500 ohm load. If you can't do that, put a 250 ohm resistor in series with the PLC input. Then 2x the scaling. A crude workaround. Maybe you could get a 4-20mA booster?

Dan:

Thanks for the response. I think in hastily posting I wasn't clear on what I encountered and what I did. I think you misunderstood your pt#2 in that the signal that was read with the fluke corresponded with the SCADA but not the Grundfos LCD. So your pt#1 and pt#2 should have the same results. You are correct pt#2 and pt#4 are the same test I am just reporting the actual value bearing in mind that your understanding of pt#2 is not correct and one was done with the acutally field reading and one was done with the test current still on. I will reiterate it so it is more clear for everyone that is taking time out to help me has a correct understanding.


Situation:
Grundfos pH/Chlorine analyser connected to Modicon 875-111 analog input card. Monitored and scaled by Wonderware SCADA

Problem:
Reading on SCADA and Grundfos LCD do not jive.

Test #1
Inserted Fluke digital multimeter in series in the circuit.
Findings #1: current reading corresponds to SCADA reading and PLC register but not with Grundfos LCD.

Test #2
Disconnected Grundfos from PLC and attached 4-20mA analyser/generator to leads. Used current test function on Grundfos to ouput current.
Findings #2: Set Grundfos test currents to 4mA and 20mA. Analyser reads 4mA and 20mA (actually 19.97mA but close enough) Therefore, assumed current output ok

Test #3
Keeping Grundfos disconnected connect the 4-20mA analyser/generator to PLC. move through 4mA, 12mA, 16mA, 20mA. Range in SCADA (also Grundfos) analyser set for 0-2 for Chlorine.
Findings #3:
SCADA screen reads 0,1,1.5, 2. Therefore, assume scaling is correct.

Test#4: Same as test #1 but with Grundfos test current still on.
Findings #4: Example at 20mA the reading on the fluke was approximately 16.85mA. Therefore, assumed some sort of distortion/signal loss and unsure of cause. Decided to post problem on plcs.net

I hope this clarifies what I encountered and what I did. Again, thank you to everyone that has and will respond. I will post the results once I have solved it. I will check back frequently to see if anyone else has offered additional suggestions.

Alaric,

Thanks for the suggestion it was good to go back and double check the literature.

Keith,

How does 500ohm max mean minimum? I am just wondering what I am not understanding.

I will hopefully, try Tom's suggestions this afternoon and report back.

Thanks again,

Tim

timothywhitson said:

Keith,How does 500ohm max mean minimum?

Click to expand...

The word 'Load' is the key. 500 Ohm max load. That means 250 ohm is a greater load.
The clarification you posted above still jives with what I'm saying.

Thanks for the clarification, Tim.

The Modicon spec for resistance of 250 ohms for a current iuput will work OK since the Grundfos can drive up to 500 ohms.

Your tests show that the Grundfos output is OK when Grundfos supplies current to a Fluke meter, which is presumably floating, and battery powered. Correct?

A floating meter will not show a ground loop.

However, when the Grunfos is connected to a multi-input AI board, the current value is not correct.

The Grundfos LCD does not agree with the Fluke reading (your test #1), indicating that the Grundfos expects to output a certain value, the LCD value, but its current output does not match the expected LCD reading.

I agree with Tom Jenkins, that this is most likely ground loop problem.

The Grundfos is a 4 wire device, powered by AC. Although the Grundfos spec states that the output is isolated from the input, but does not tell how the output is isolated from its power supply or earth ground or any ground in the system.

Like Tom explains, it is typical on multiinput AI's to have the first one work OK, add a second, it works OK, add a third, it works OK, but add a fourth (or whatever number) and suddenly the total common mode seen by the card exceeds its rating and the added input won't read properly. However, common mode problems usually saturate the inputs, driving them full scale up or down. Or the common ground in the system (4 wire transmitters are notorious for not isolating their outputs) can create ground loops that suck or add current to your loop signal.

Tom's test is correct - disconnect the other analyzer devices, then connecting the Grundfos pH by itself and it will probably work OK. If it does, you've proved your need for isolation between the Grundfos and the Modicon. A commercial 4-20mA-in-to-4-20mA-out signal loop isolator will fix you up.

Dan

danw said:

The Modicon spec for resistance of 250 ohms for a current input will work OK since the Grundfos can drive up to 500 ohms.

Click to expand...

A dead short is zero ohms. Open is infinity. You need to think backwards when looking at this spec.

Take a 500 Ohm LOAD. Connect another 500 Ohm LOAD in parallel. Now you have a 250 Ohm LOAD. Connect 2 more in parallel, and you will now have a 125 Ohm LOAD.

It's basic Ohms law.

Keith, I think you're thinking of this as a 2 wire loop powered current loop, whereas the Grundfos is a 4 wire device.

Admittedly, in 2 wire, loop powered current loop, the symptom of insufficient loop voltage is a diminished current output, lower than expected, or working up until a certain point, and then stalling at a current value.

A lower than expected reading is what is happening, but what is the likelihood that the Grundfos internal power supply is dysfunctional and cannot supply sufficient voltage to drive the current? It's possible, but not very likely.

The situation does have the earmarks of a ground loop. The test is to connect the Grundfos by itself to the AI card, without any other devices.

If the Grundfos works OK connected by itself on the Modicon AI, then it proves the ability of the Grundfos to drive the circuit, in the absence of a ground loop produced by additional devices.

If it can't drive the AI, then it is likely a faulty output.

If it can drive the AI, then ground loop solution is an isolator.
Dan

keithkyll said:

The word 'Load' is the key. 500 Ohm max load. That means 250 ohm is a greater load.
The clarification you posted above still jives with what I'm saying.

Click to expand...

keith,

Your thinking works for voltage but not for current. The spec for a current transmitter is the amount of load (resistance) it can push its current through. For 500 ohms, that is the upper limit, anything greater the transmitter will not be able to provide the current. So 250 ohms would be less load (for a current device).

Voltage transmitters want a high resistance load ( it doesn’t have to work as hard). High resistance loads for voltage devices means less current draw from them. Low resistance loads would mean higher current (if to high, low load resistance) the voltage device might not have the power to drive the load.